In a large-scale system such as an industrial machinery system or manufacturing facilities, input/output devices such as a large number of sensors and a large number of drivers, actuators, and the like are placed at various points. Meanwhile, a control device such as a computer or a sequencer is provided to control and monitor those devices.
If the sensors are photo-interrupters or the likes, on/off data is transmitted to the control device through transmission channels. If the sensors are to detect temperature, voltage, or the like, data generated by A/D converting to the on/off data is transmitted to the control device through transmission channels. On the other hand, control data is transmitted from the control device to drivers, actuators, and the like through transmission channels, so as to control motors, cylinders, and the like.
If one cable is used at each installation point of the sensors and drivers to form the above transmission channels, an extremely large number of cables are used in total. Therefore, many problems are caused, as it is difficult to reduce the size of the system and maintain the system.
On the other hand, serial bus transmission systems have been known. A serial bus transmission system is a network in which slave nodes and a master node that controls the bus are connected in a multi-drop manner to a bus line formed with one to three signal lines (see Non-Patent Document 1).
In a serial bus transmission system, the operation of each node is determined by a combination of a signal voltage and its transition state, and each node carries out a network control flow by following predetermined procedures.
As for the network control methods, there have been a bus arbitration method by which a collision can be avoided, and control can be established even when each node arbitrarily accesses (Non-Patent Document 1, Patent Document 1), or a cyclic method by which time slots that enable sending are sequentially allotted to respective nodes in a fixed manner (Patent Document 2). If a serial bus transmission system is used as the transmission channels, the number of cables can be dramatically reduced.
In an industrial machinery system or manufacturing facilities, however, a large amount of noise is generated. Due to the noise, an abnormality is caused in the network control flow. If the noise lasts over a long period of time, the network control flow is disturbed to a great extent, and the large-scale system might have a fatal error in an operation.